(A) Field of the Invention
The present invention relates to a package structure of a light emitting diode (LED) for a backlight, and more particularly, to a long-wavelength LED for increasing the color rendering index of a backlight.
(B) Description of the Related Art
An LED is a semiconductor device, and can serve as a light source with a small volume effectively emitting high-intensity light. Moreover, the light of the LED has a superior mono-color peak in its spectrum. If a white light is generated by LEDs, several color lights emitting from the LEDs need to be mixed. A simple way to get a white light is to simultaneously use red, blue and green LEDs. However, such a white light, a mixture of red, blue and green lights, is disproportionate in its spectrum. A proportionate white light is obtained by mixing various color lights in a complicated optical manner. Furthermore, the brightness of an LED varies with the temperature of the LED semiconductor. That is, the mixed white light changes when the temperature of the LED rises. In addition, the working life of one LED is different from that of another. Accordingly, when one color light LED fails, the change in the mixed light is obvious so that a viewer easily sees an unharmonious light dot. In view of the above, a superior technology for uniformly mixing white light is important.
An existing LED device emits white light that comprises blue light originally generated by an LED mixed with yellow light emitted from fluorescent powders excited by the blue light. However, such a light mixture for the application of a backlight is insufficient in the intensity of red light so that its color rendering index is lower. That is, a viewer is likely to sense that the color shown on the screen lacks red. Therefore, Shimizu et al. put forth a method claimed by U.S. Pat. No. 6,577,073 for generating white light with various color systems by mixing blue light, red light and fluorescent powders. The fluorescence emitted from the excited fluorescent powders has wavelengths ranging between wavelengths of red and blue light. In this regard, the resulting white light is a warm color light. A method for adjusting the white light is to simultaneously tune the wavelength of the blue LED, the intensity of the blue light, the wavelength of the red LED, the intensity of the red light and the spectrum of the fluorescence. However, the adjusting method is too complicated. Furthermore, with regard to the package structure of the LED, the red LED can absorb the blue light so as to reduce the output of the blue light. A backlight using such an LED device appears to the viewer to be weak in the component of the blue light. The efficiency of the integral light output is reduced because the fluorescent powders block the red light.